This invention relates to nuclear instrumentation used ot monitor the integrity of fuel rods of nuclear reactor fuel assemblies.
Reference is hereby noted to a copending application entitled "A Device and Process to Differentiate Gross Exposure of Nuclear Fuel from Leaking Fuel Rods" to Kadambi.
Nuclear reactors contain a fuel core which is a grouping of fuel assemblies each of which has a plurality of fuel pins. A fuel rod is a crylindrical tube which contains nuclear fuel pellets. The metal side of the tube separates the fuel pellets from reactor coolant which flows over the surface of the tube or cladding.
A penetration of the cladding termed a fuel failure, may allow fission fragments, particularly gases, to escape from the fuel pin, and may allow coolant/fuel contact.
Nuclear reactors are expected to experience fuel failure in spite of rigorous quality control and conservative operating procedures. Most of the failures result from pin-hole cracks in the cladding and/or end plug welds. Such failures are detected by analysis of fission-gas outside the core (e.g., in the reactor cover gas) and by observation of delayed neutron precursors in the reactor coolant.
The significance of fuel failures arises due to three factors as follows:
(A) Safety: Although failed fuel which only leaks gas is of negligible safety significance, there is concern that the breach in the fuel could become large enough to allow ingress of coolant into the pin, or escape of some fuel particles. In Liquid Metal Fast Breeder Reactors, sodium reaction with fuel material generates a product which is less dense than fuel, causing an increase in volume and sometimes an increase in the breach size. The safety concern from fuel swelling or fuel release occurs because of possible reduced heat transfer from the pin, or the remote possibility that the particles could accumulate to form a heat-generating flow blockage within the core. PA0 (B) Plant Maintenance: Some of the fuel material entering the coolant could be transported away from the core and deposited in any part of the piping, heat exchangers or the pumps. Such deposits could complicate maintenance operations in and around the components. PA0 (C) Economics: Due to the inevitability of fuel failures, the availability of the reactor is significantly improved by operating with a limited number of failed elements. Thus, if the safety and maintenance concerns can be resolved, there is considerable incentive to operate with failed fuel.
One of the problems faced by reactor instrumentation is to detect and monitor failed fuel in such a manner that safe operation of the reactor is not impaired. This problem can be solved by having the ability to observe changes in failed fuel so that significant increases in exposure of fuel to coolant are clearly and reliably annunciated. Relatively isolated and random cladding failure in nuclear fuel is innocuous. If fuel exposure and fuel release occurs, a possible mode of propagation of the failure may be postulated, although certain experience indicates that cladding failures remain localized events. Analyses indicate that small heat generating blockages can be tolerated without jeopardizing safety. If such a blockage can be reliably detected, reactor safety can be further assured by precluding growth of a blockage beyond tolerable limits.
Consequently, it is desired to provide a method for the detection of fuel failure, such method being capable of differentiating between gross failures and small leaks which result in only fission gas leakage.